Handle the Pair, Regex, and IntSeq types during codegen and evaluation

[HT154]

No description provided.

[bioball]

We decided to not have Pair because:

1. We'd like to turn these into Swift tuples if possible
2. Pair is not meant to store config values, and generally we don't expect them to cross the Pkl <> Swift boundary.

The same logic can be applied to IntSeq and Regex (prefer using native types than bringing our own).

I'm not totally opposed to merging this, but I'd like for more users to flag this as an issue before doing anything here.
It's easier to leave this as unsupported for now and introduce later, than to introduce a breaking change (e.g. if we introduce Pair then change to tuples).

[HT154]

I generally agree with the concerns here and I'm fine not shipping this.

Getting tuples to work here is likely challenging. I spent some time poking at it, but there are obstacles for sure. I think it would be necessary to decode into a Pair struct and then extract the tuple from there, which would mean always synthesizing init (and possibly other methods) in affected generated code.